The background tropospheric ozone concentration is depleted in the vicinity of urban areas due to the reaction of ozone with nitric oxide which is emitted by a range of urban sources. A model has been developed to simulate the three-dimensional structure in ozone and nitrogen oxides (NOX) concentrations in the boundary-layer within and downwind of urban areas. This is based on the boundary-layer Lagrangian column model of ApSimon et al (1994)*. The underlying photochemical processes of NOX and ozone, as well as emissions and deposition to the ground are simulated. The model treats the stability dependent vertical mixing in the boundary-layer with detail, dividing the boundary-layer into 33 levels, with greater resolution near the surface where the highest concentration gradients occur. The model simulates the diurnal variations in mixing using a combination of mechanical and convective turbulence.
The model is applied over a 100x100km domain, containing the city of Edinburgh (with a diameter of 12km), to simulate the city-scale processes of pollutants and to allow comparison with monitoring stations within and downwind of the urban area. Results are presented, using averaged wind-flow frequencies and appropriate stability conditions, as well as individual episodes, to show the extent of the depletion of ozone by a city. A landuse array has been created for input to the model with spatially and temporally variable emission and deposition values. The long-term average spatial patterns in the surface ozone and NOX concentrations over the model domain are reproduced quantitatively. One-dimensional trajectories through the city have been run under northerly and southerly wind regimes to allow comparisons to be made with data recorded from monitoring sites that lie on a north-south transect through the city - from an urban, through a semi-rural, to a remote rural location. Diurnal and annual (inter-seasonal) variations of ozone concentrations have also been modelled for the city-centre site, where the origin of the wind should not influence the extent of the ozone depletion.
* ApSimon, H.M., Barker, B.M., Kayin, S., (1994). "Modelling studies of the atmospheric release and transport of ammonia in anticyclonic episodes." Atmos. Environ. 28, pp665